Fix theme propagation for children of top level controls and windows

[YuriSizov]

Fixes #36103 for master. The issue was in the logic that assigned theme owner. For some reason, probably in error, theme owner was never assigned if the direct parent was a control that was marked as top level. Not entirely sure, why this only affected editor plugins (at least in 3.x), but not running projects.

A further regression witnessed by @KoBeWi in the comments was because Window was missing this logic entirely.

I've also cleaned up related code a bit, since we don't need a node reference anymore. I'm not very happy with the theme owner / theme owner window logic that we have, and the duplication that we have. It's too convoluted for such a simple system. But I decided to focus on fixing the issue, and didn't do any refactoring.

A slightly modified repro for master:
bug-theme-dialogs-4.0.zip

[reduz]

This looks good to me. I guess not much we can do that code gets complicated with windows, but it does what it has to do.

[akien-mga]

Thanks!